Kotlin grouping, mapping and so on - kotlin

Question

I know that a lot of this topics exists but I don't understand this topic at all. And an exact explanation of what is happening for a beginner is needed.
I have a list of persons:
val person1 = Person("A", 8, 24,"darts")
val person2 = Person("A", 8, 24,"football")
val person3 = Person("A", 2, 24,"basketball")
val person4 = Person("B", 8, 24,"skiing")
val person5 = Person("B", 1, 24,"snowboard")
where
data class Person (val street: String, val number: Int, val age: Int, val hobby: String){
override fun toString(): String {
return "$street $number $age $hobby"
}
}
And now to sum up. If I use a groupBy the result will be Map with key, and values Am I right?
Where keys are the atrributes on which I am grouping by like:
var grouping = list.groupBy { it.street }
result is:
{A=[A 8 24 darts, A 8 24 football, A 2 24 basketball], B=[B 8 24 skiing, B 1 24 snowboard]}
And now I would like to group by multiple fields like street and number. How to do this?
And moreover I would like to specify a LIST (I have to do some kind of projection from this map?) of hobbies which people have under adresses and numbers. For example under adress A number 8 i have a list of (darts,football).
#Edit
Do I have somehow to divide this list as 2nd object class?

Answer 1

Yes, groupBy returns a map where the keys are whatever was returned by the lambda and values are lists of items from the original list.
To group by a pair of things, you need to be able to have a key that represents all those things and be unique. The probably means another data class, or maybe you could concatenate the as a String.
data class Address(val street: String, val number: Int)
val grouping = list.groupBy { Address(it.street, it.number) }
And to make the values of the map simply the hobby, you can use mapkeys:
val addressesToHobbies = grouping.mapKeys { it.map(Person::hobby) }